686. Welsbach Gas Lamp With Bull's-Eye Lens

Welsbach Gas Lamp With Bull's-Eye Lens. In place of the bull's-eye condenser, and where gaslight is used as the illuminant, the Welsbach gas lamp with metal chimney and iris diaphragm, and bull's-eye lens, as shown in Illustration No. 125, page 344, may be used. This illustration represents an ordinary Welsbach lamp fitted with a metal hood which cuts off all unused light, and the lens barrel, being supplied with an iris diaphragm, concentrates all the light on the object. By the adjusting of the iris diaphragm you can illuminate the structure of the mantle of the lamp. Where no diaphragm is used a ground-glass may be placed between the burner and the microscope in the same manner as the color-screen in Fig. 3, of Illustration No. 124. Without the ground-glass or diaphragm the structure of the mantle would appear in the picture.

687. Reflected and Transmitted Light - After the question of illuminant has been settled, we next have the nature of the object to be photographed to contend with. While most of the subjects selected to be photographed are sufficiently transparent to be illuminated by light transmitted from the mirror, yet some subjects, such as minerals, shells and plants, are so dense in their structure that direct light such as would be used with the instrument arranged horizontally, or the light reflected from the mirror below the stage will not pass through them and, therefore, will need to be photographed from their reflected surface - in other words, with the light falling upon the object instead of transmitted through the object. The latter mentioned class of subjects requires a low-power magnification, and for the best of results the bull's-eye condenser should be employed. This condenser is designated by the letter L of Fig. 1, Illustration No. 124, and is shown in its proper position for opaque illumination in Fig. 1 of Illustration No. 126. You will also observe the gaslight is arranged at an elevation above the stage. This is necessary to bring the ray of light through the bull's-eye condenser onto the surface of the object on the slide.

688. Additional important parts to this outfit are designated in Fig. 1, Illustration No. 124, as follows:

G - Coarse adjustment rack H - Cardboard or tin reflector I - Cedar oil bottle in nickel case J - Yellow filter screen K - Hand-focusing glass L - Bull's-eye condenser M - Welsbach light N - Camera cone thumb-screw O - Camera cone P - Camera box Q - Coarse adjustment R - Fine adjustment S - Objectives T - Abbe condenser U - Reflecting mirror V - The stage

689. The bull's-eye condenser, when employed on opaque subjects, is used in a manner to concentrate a beam of light upon the surface of the object; while when used for slides of transparent subjects the bull's-eye condenser is used to concentrate light upon the mirror or below the stage, and the mirror in turn, reflecting this light through the object or slide. H is a small piece of tin or white cardboard, bent in a semi-circular shape placed behind the slide to further assist in reflecting the light upon the object.